The present invention relates to a method and apparatus for determining the profile of a coating layer. A web is brought to a coating machine, and at the coating head of the machine, a coating slip is applied to a surface of the web. The profile of the coating layer applied to the web is determined.
When coating paper or paperboard, a coating slip is first applied to a surface of a moving web, after which the surplus coating slip is removed and the surface of the coating layer is evened. Along with the coating slip, moisture enters the web; therefore, the excess moisture in the coating layer and the web is removed by drying. The coating machine can be located directly after the base paper machine, in which case the process is called on-machine coating. The coating machine can also be a separate device, to which the wound base paper rolls are taken and where they are unwound and coated, separately from the rest of the paper machine. This is called off-machine coating.
The web is dried by the drying part of the coating machine. The most common drier types include the infrared drier, the blast drier, and the drying cylinder. In the infrared drier, infrared radiation is emitted from the drier with the intention of inducing the highest intensity of radiation in wavelengths, where the absorption of radiation by water is high. In that case, the heating effect of the drier can be directed at the moisture among the coating as accurately as possible, whereupon the drying effect is the highest. In the blast drier, hot air is used to dry the web. In cylinder drying, for example, the web travels against a cylinder that is heated with steam, so that heat is transferred by convection from the cylinder surface to the web.
The drying of the coating slip layer can be divided into four different main stages. The first stage is a heating stage, during which the web temperature is quickly increased, generally, by the first infrared drier after the coating head. The web heats enough for the mass transfer, i.e., the rate of evaporation to be well balanced with the heat transfer. During the heating stage, moisture is absorbed from the coating layer into the base paper, the fibres of which swell up.
The second stage is the so-called stage of smooth evaporation, when all the heat that has been transferred to the web is consumed by drying. Generally, blast driers carry out the drying in the second stage, whereupon the temperature of the coating settles into balance with the moisture and temperature of the ambient air.
The third drying stage is the so-called first stage of decreasing evaporation, whereupon, when the surface of the coating layer dries, some of the capillaries that carry water to the surface are emptied and the evaporation is slowed down. In the third stage, the temperature of the coating starts a considerable increase.
The fourth and last drying stage is called the second stage of decreasing evaporation. As the capillaries no longer carry water to the surface, the surface of the coating layer is now completely dry. Therefore, the zone of evaporation moves inside the coating layer. The surface temperature of the coating layer increases very quickly. In the last drying stage, the coating is dry enough for the drying to be carried out by drying cylinders that touch the web.
When assessing the coating result, the coat weight is the most important indicator. The coat weight indicates the amount of dry matter of the coating slip on the paper in grams per square meter. The intention is to keep the profile of the coating layer both in the machine direction and the cross machine direction as even as possible with the aid of closed loop controls. In the machine direction, the profile of the coating layer is controlled, for example, by changing the load of the doctor blade or the doctor bar. In the cross machine direction of the web, the profile of the coating layer is controlled, for example, by locally changing the load of the doctor blade or the doctor bar in places, where the coat weight is too high or too low. The prerequisite for the profile control in both the machine direction and the cross machine direction is accurate measuring of the coat weight.
In measuring the coat weight, generally, measuring frames are used, comprising a slide provided with sensors, moving across the entire width of the paper web. The coat weight can be determined by measuring with sensors, for example, the absorption of beta radiation, the absorption or the fluorescence of X-radiation or the absorption of infrared radiation in the coating. Generally, the sensors measure one point only; therefore, the slide must cross the web to measure the profile of the coating layer along the entire width of the web.
The properties of the sensors set limitations to the speed of the measuring frame slides. The low power of the radiation sources in particular limits the slide speed in the case of sensors that are based on radiation. For the current sensors, the propagation speed of the slide is about 20 to 40 cm per second, whereupon it may take as much as one minute for the slide to cross a wide web, i.e., to perform one scanning. As the slide proceeds very slowly in the cross machine direction compared with the web speed in the machine direction, the method distorts the measuring results. While the sensor crosses the web once in the cross machine direction, the paper has travelled as much as over a kilometer in the machine direction. Normally, the transverse profile of the coating layer is calculated as the mean value of several scans, whereupon the adjustment of the transverse profile of the coating layer is slow and has an effect on long-term changes only.
Another problem in slow scanning is the effect of the differences of the coat weights in the machine direction on the transverse measurement of the coat weight. Any changes in the coating profile in the machine direction are serious interferences from the point of view of the transverse profile. Changes in the machine direction are often greater than any irregularities in the transverse profile, making the profile measurement unreliable, unless the changes are compensated for.
The goal of this invention is to provide an entirely novel method and arrangement for measuring the profile of a coating.
The invention is base on the fact that the profile of the coating applied to the web is indirectly determined or defined by measuring the surface temperature of the coating layer, for example, by a thermographic camera or an infrared pyrometer after the coating head. The measurement can be performed before the first drier or after any drier. Changes in the coat weight also cause changes in the temperature of the coating layer. When needed, the temperature of the base web can be measured before the coating head, whereupon the coating profile of the web can be determined with the aid of the changes in the difference of the temperature of the coating layer and the base web, or with the aid of heat balances. The method is well suited to determining the coating profile both in the machine and the cross machine direction.
The invention offers significant benefits.
The thermographic camera or the infrared pyrometer used as the measuring device has a large measuring angle, whereupon the surface temperature distribution of the coating and, on the basis of that, the profile of the coating layer can be quickly determined. Because of the quick determination of the coating layer profile, the coat weight to be applied at the coating head can be quickly adjusted. The invention can also be used to correct any profile defects in the base paper. In addition, the measuring equipment according to the invention is simple and easy to install both in new and existing coaters.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are intended solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.